The present invention relates generally to sensors. More specifically, the present invention relates to a sensor for determining spin rate of a rotating body within an external magnetic field.
In military applications, it is generally agreed that the number of revolutions that a spinning projectile makes is an accurate indicator of its range. The distance that a projectile will travel linearly per revolution is a constant. Theoretical analysis shows that a xe2x80x9cperfectxe2x80x9d spin counting fuse will measure the range of a projectile better than a xe2x80x9cperfectxe2x80x9d time counting fuse. Variations in muzzle velocity will not affect the ranging of a spin counter because distance is now calculated independently from velocity.
The advantages of developing and implementing a spin counting fuse are several. The fuse would be inherently more accurate than an electronic timing (ET) fuse. The fire control system would not need to adjust the fuse setting to account for round-to-round velocity deviations. This will simplify the system by eliminating those components in the fire control and the weapon that measure and correct for differences in projectile speed. The overall benefit is a cheaper, and more reliable system.
A prior art method of determining spin rate is the use of magnetically-sensitive coils to sense the earth""s magnetic fields. As the coil rotates about the projectile""s spin axis, the sensor responds to the earth""s magnetic flux density to count rotations of the projectile. A drawback of this approach is the size of the coils needed for certain applications where the sensor needs to count spins onboard a very slow turning projectile, such as the KE anti-tank round. Coil sensors are speed sensitive, i.e., the slower the speed the more coil windings are required. Therefore the coils must become larger for slower speeds, effectively precluding the use of these sensors in smaller caliber rounds.
There is a need therefore, for an improved method of determining the spin rate of a rotating body within an external magnetic field, e.g. a magnetic field of the earth.
The present invention offers advantages and alternatives over the prior art by providing a sensor using the phenomenon of giant magneto-impedance (GMI) to determine the spin rate of a rotating body, such as an artillery shell, in an external magnetic field, such as the magnetic field of the earth. The present invention eliminates the need for the fire control system of a weapon to account for round to round velocity deviations. Moreover, by eliminating the use of speed sensitive coils, the present invention may be used on slower traveling, smaller caliber rounds which was not possible with prior art sensors.
These and other advantages are accomplished in an exemplary embodiment of the invention by providing a GMI sensor for determining spin rate of a rotating body within an external magnetic field. The sensor comprises an oscillator generating an alternating current (AC) drive signal at an oscillator output terminal, a voltage divider circuit and a signal processing circuit. The voltage divider circuit electrically connects the drive signal to a reference ground by providing a first terminal AC coupled to the oscillator output terminal through a coupling capacitor, and a second terminal electrically connected to the reference ground. The voltage divider circuit further includes a GMI fiber having a GMI junction terminal. The fiber is disposed in fixed relation to the rotating body such that impedance of the fiber is modulated by the external magnetic field to provide a modulated, e.g., amplitude modulated, drive signal at the GMI junction terminal. The signal processing circuit is electrically connected to the GMI junction terminal and processes the modulated drive signal to provide an output signal indicative of the spin rate of the rotating body.
In an alternate of the invention, the signal processing circuit further comprises a rectifier having an input terminal in series connection with the GMI junction terminal and a low pass filter in series connection with an output terminal of the rectifier. The signal processing circuit rectifies and filters out the drive signal to provide a data signal at the output terminal of the rectifier. The output terminal of the rectifier, i.e., the data signal, is electrically connected to either of an inverting and non-inverting terminal of an amplifier circuit. A reference voltage circuit generates a reference voltage which dynamically tracks amplitude variations in the drive signal due to outside factors such as temperature. The reference voltage is electrically connected to the other of the inverting and non-inverting terminal of the amplifier circuit. The data signal is amplified by the amplifier circuit to provide the output signal.